1. Field of the Invention
The present invention relates to a dielectric filter for use in mobile communication apparatus, such as automobile telephone sets and portable telephone sets.
2. Description of the Related Art
FIG. 5 shows an equivalent circuit of a conventional band elimination filter, which is constituted by, for example, resonators R1 and R2, trap capacitors Ct and a coupling capacitor Ce. Such a conventional dielectric filter may have the configuration shown in FIG. 4. In FIG. 4, two block-shaped dielectric resonators 1 and a capacitor substrate 11 with capacitor electrodes formed on two surfaces thereof are mounted on a base plate 10. Input/output electrodes and grounding electrodes are formed on two surfaces of the base plate 10. Each of the dielectric resonators has an inner conductor 3 which is electrically connected to the capacitor substrate 11 by a metal connection terminal 20. An outer conductor 4 of each of the dielectric resonators is electrically connected to the grounding electrodes on the substrate 10.
In the above-described conventional dielectric resonator, a single resonator hole 2 is formed in a substantially rectangular parallelepiped-shaped dielectric block 1 in such a manner that it passes through a pair of opposing end surfaces. An inner conductor 3 is formed on the inner peripheral surface of the resonator hole 2, and an outer conductor 4 is formed on five plane surfaces of the block 1 except for a plane 1a (hereinafter referred to as an open-circuited end surface) thereof, the plane 1a constituting one of the two planes to which the resonator hole 2 is opened. The inner conductor 3 is separated (open-circuited) from the outer conductor 4 at the open-circuited end surface 1a, and is electrically connected (short-circuited) to the outer conductor 4 at a plane 1b (hereinafter referred to as a short-circuited end surface) of the block 1 which constitutes the other of the two planes to which the resonator hole 2 is opened.
The trap capacitors Ct shown in FIG. 5 are formed by capacitances between the capacitor electrodes formed on the upper and lower surfaces of the capacitor substrate 11, and the coupling capacitor Ce is obtained by the capacitance formed in a gap between the capacitor electrodes on the lower surface of the capacitor substrate 11.
These conventional dielectric filters require many parts, including a plurality of dielectric resonators, a capacitor substrate used to obtain the capacitance constituting a filter circuit, a base plate on which various electrodes are formed and on which the dielectric resonators, the capacitor substrate and so on are mounted, connection terminals for connecting the dielectric resonators to the capacitor substrate and so on. Further, these parts must be soldered for assembly. Thus, a reduction in the size of the entire filter is difficult to obtain, the cost of parts is increased, and the number of manhours required for the manufacturing process and hence the production cost are high.